Author Topic: Bosch Dishwasher SMU68M05AU - E24 error [FIXED]  (Read 7327 times)

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Offline Lorenzo_1

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Bosch Dishwasher SMU68M05AU - E24 error [FIXED]
« on: May 01, 2018, 12:22:59 am »
Thought a short post on this repair might be helpful, as Error Code E24 (Check Water) seems to be commonplace but none of the suggested fixes I could find online was helpful in this case.  Symptom was failure to drain water fully during the wash cycle, resulting in a program halt. Common suggested fixes are checking for blockages in drain hoses/outlets and inlet hose, cleaning the filter system, checking for debris blocking the drain pump impeller etc.  In this case I tested and cleaned all these systems but the fault persisted.  There is a manual for similar Bosch models available online - but it takes some diligent searching to find (thought it best not to post link here). That was helpful in describing how the quite complex system works and giving some resistance tests for pump motors and solenoids, but gives no guidance on error codes. Seems the microprocessor logs error signals but they're only accessible with proprietary Bosch hardware and software.

Resorted to oscilloscope tests to see whether control board was driving the pumps OK.  Signals there all looked OK (pulsed square waves), but it became apparent that the program stop was not correlated with the start or end of pump drive signals.  Instead, it happened only when the water conservation system was trying to store water in the reservoir attached to the LHS of the machine.  That pointed to the flow sensor on the reservoir - this has an impeller adjacent to the flow channel with magnet that opens/closes a microswitch (photo attached). CRO quickly showed the switching impulses from it were intermittent, suggesting the switch was failing. $45 later problem solved. High price for such a small board, but cheaper than a new dishwasher or a service call. Searched for a replacement microswitch but more trouble than it was worth.

Another task to do while you have the system open is to clean the flow channels and float (drainage hose ventilation valve) shown in the reservoir photo.  They get gunked up badly and float doesn’t operate.  I removed the tank, flushed it with water and then hot concentrated dishwasher detergent, leaving it to soak.  Note that you have to remove two plastic screw fittings (inlet & outlet) from inside the dishwasher before the tank will come free. To shift the last of the caked on goo I had to resort to careful use of an air chisel to vibrate it free!  That worked fine – though it took quite some time.
 
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Offline ebastler

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Re: Bosch Dishwasher SMU68M05AU - E24 error [FIXED]
« Reply #1 on: May 01, 2018, 05:58:28 am »
Thanks Lorenzo; nice summary of your repair!

For future reference: There is a great collection of repair briefs for dishwashers, washing machines etc. from Bosch/Siemens/Neff here:
https://forum.teamhack.de/filebase/file-list/

Specifically, disassembly and most common repairs of their dishwashers:
https://forum.teamhack.de/filebase/download/19/

(In German, I'm afraid, but worth the effort. BTW, while there is some hacking involved in some of the repairs, the site owes its name to Mr. Hack's last name. ;))
 

Offline Lorenzo_1

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Re: Bosch Dishwasher SMU68M05AU - E24 error [FIXED]
« Reply #2 on: May 01, 2018, 06:13:42 am »
Thanks for the links - there's some really useful stuff in there. Didn't search on non-English sites. My German's non-existent but Google translate helps a bit and the photos are often self-explanatory.  My antivirus detected a malicious page in one of the pdf files on the first link - not sure if it was real or not. Most were OK. I'm sure some of my other European appliances will decide to play up as soon as I turn my back. 
 

Offline ornea

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Re: Bosch Dishwasher SMU68M05AU - E24 error [FIXED]
« Reply #3 on: May 16, 2020, 05:56:09 am »
Thanks for your post Lorenzo_1.  I though I would add my experience and ask some questions. 

My machines (SMS63M08AA which looks similar) behavior at start of cycle, it will run the siphon pump, then run the recirc pump and switch back and fourth eventually throwing an E24.  If I open and close the door at the right point the cycle will begin but again fail later in the cycle (sometimes)

I have spent many many hours with E24 finding many red herrings that I was convinced was the issue, some of these follow.

1. PCB edge connectors seem problematic.  Measuring winding resistance of drain pump for example would have the reading bounce around if the wires were ever slightly moved.  So I cut some of them off and soldered direct. No fix

2. Flood water detector micro switch also had the Resistance values bounce around each time it was operated.  Solution, dismantle and clean micro switch. Now both NC and NO contacts shows ~0.2 ohms  when operated. In this case I don't think it was the contacts that were high resistance but where the toggling pole connects to the frame its in. Confident this was the cause but no fix.

3. Flow channel reed switch. I observed if I  operate the reed switch by introducing a magnet at the right part of the startup cycle it begin filling and the cycle would start.  If I do this when the system has not requested a fill (ie the solenoid is off) or do it too much it generates E16, understandable.  My reed switch appears to be operating ok. I did suspect it but tests ok.  I also noticed that when the system is at rest the reed is Normally Closed, I arranged a magnet to change this to Normally Open and the first test cycle began without a hitch. i thought i was onto something but next cycle it failed.

My Questions:-

1. I wanted to observe the drive signals to the pumps with a scope but not sure how to connect the ground lead.  I also want to look at the supply rails but again concerned about where I should connect the ground clip.  How did you do this?

2. In the flow path you mention a "drainage hose ventilation valve", is that a simple mechanical structure or is there a sensor in there.  Mine is gunked up. Today I am going to clean but curious if there is a sensor there to detect a blockage.  I dont thinks so. I cant see wires or any sensor there but is gunked so hard to see.

3. How do you enter a Test Cycle or Test Program so I dont have to wait 2 hours to know if it ran ok.

I will continue to update this thread particularly if I find the cause.
« Last Edit: May 16, 2020, 08:25:47 am by ornea »
 

Offline ornea

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Re: Bosch Dishwasher SMU68M05AU - E24 error [FIXED]
« Reply #4 on: May 16, 2020, 07:59:55 am »
Lacking intelligent moves I decided to replace the inflow reed switch that tested 100% using multimeter and magnet with another that tested ok but with contact resistance around 1.5ohms.

To my surprise the first rinse cycle started and completed without a hitch  ::) but the first normal cycle error'ed E24  |O.  However the next attempt at a normal cycle proceeded ok. :-//

So 2/3 aint bad. but not right.

I have been pondering:-
1. how does this machine know when the side tank reservoir is full (does it count pulses on the inflow sensor, i would think not accurate enough for that)  and
2. how does it know when the sump is full enough.  At the start of a cycle it dumps the side tank into the sump, starts recirc and then adds more water.  How does it know when to stop.  I see no sensors. Perhaps it measure back EMF from the recirc pump.  There is a turbidity sensor but that is submerged after about a cup of water has entered the system.
 

Offline xavier60

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Re: Bosch Dishwasher SMU68M05AU - E24 error [FIXED]
« Reply #5 on: May 16, 2020, 08:24:39 am »
I fixed my brother's dishwasher a long time ago. It could have been the same model and I can't remember the error code, it was pump related. I found a faulty dual opamp in a pump related area of the board.
Hioki AS100D vom, HP 54645A dso, Fluke 87V dmm, AN8008 dmm, Agilent U8002A psu,  FY6600 function gen, New! Brymen BM857S-(With Battery)
 
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